4 to 20 Ma Calculation Formula

4 to 20 mA Scaling Calculator

Range Setup

Calculate Current (mA)

Calculate Process Value

function calculateMA() { var minP = parseFloat(document.getElementById('minProcess').value); var maxP = parseFloat(document.getElementById('maxProcess').value); var val = parseFloat(document.getElementById('inputValue').value); if (isNaN(minP) || isNaN(maxP) || isNaN(val)) { alert("Please enter valid numbers"); return; } if (maxP === minP) { alert("Max and Min range cannot be the same"); return; } // Formula: mA = ((Value – MinP) / (MaxP – MinP)) * (20 – 4) + 4 var result = ((val – minP) / (maxP – minP)) * (20 – 4) + 4; document.getElementById('maOutput').innerText = result.toFixed(3) + " mA"; document.getElementById('maResult').style.display = "block"; } function calculateProcess() { var minP = parseFloat(document.getElementById('minProcess').value); var maxP = parseFloat(document.getElementById('maxProcess').value); var ma = parseFloat(document.getElementById('inputMA').value); if (isNaN(minP) || isNaN(maxP) || isNaN(ma)) { alert("Please enter valid numbers"); return; } if (maxP === minP) { alert("Max and Min range cannot be the same"); return; } // Formula: Value = ((mA – 4) / (20 – 4)) * (MaxP – MinP) + MinP var result = ((ma – 4) / 16) * (maxP – minP) + minP; document.getElementById('processOutput').innerText = result.toFixed(3); document.getElementById('processResult').style.display = "block"; }

Understanding the 4 to 20 mA Calculation Formula

In industrial instrumentation, the 4-20 mA current loop is the dominant standard for transmitting sensor information. It is used because current does not drop over long wire distances (unlike voltage) and the 4mA "Live Zero" allows the system to detect a wire break (0mA).

The Linear Scaling Formula

To convert a process variable (like temperature or pressure) to a milliamp signal, we use linear interpolation. The standard formula is:

Output (mA) = [((Value – Low Scale) / (High Scale – Low Scale)) × 16] + 4

Conversely, to calculate the process value from a measured current, use this formula:

Process Value = [((Current mA – 4) / 16) × (High Scale – Low Scale)] + Low Scale

Key Parameters

• Low Scale: The process value at 4mA (e.g., 0 Bar).
• High Scale: The process value at 20mA (e.g., 10 Bar).
• 16: The span of the current (20mA minus 4mA).
• 4: The offset (the starting current).

Practical Example

Imagine a pressure transmitter with a range of 0 to 500 PSI. If you measure a signal of 12 mA, what is the pressure?

1. Subtract the offset: 12mA – 4mA = 8mA.
2. Divide by the span: 8mA / 16mA = 0.5 (this means the signal is at 50% of the range).
3. Multiply by the process span: 0.5 × (500 – 0) = 250.
4. Add the low scale: 250 + 0 = 250 PSI.

Why use 4mA instead of 0mA?

The "Live Zero" at 4mA serves two critical purposes:

1. Fault Detection: If a wire breaks, the current drops to 0mA. If the scale started at 0mA, the controller couldn't distinguish between a "0% reading" and a "broken wire."
2. Powering Devices: Many transmitters are "loop-powered," meaning they draw their operational power from the loop itself. The constant 4mA provides a minimum amount of power to run the sensor electronics even when the signal is at its lowest point.